An internal combustion engine in which the surface area of the portion for generating squish has been increased is proposed in Japanese Patent Laid-Open Publication No. 2005-23831, for example. A valve-operating mechanism for an internal combustion engine in which the intake and exhaust valves do not overlap in the vicinity of top dead center when the exhaust stroke has been completed is proposed in Japanese Patent Laid-Open Publication No. 2000-73803, for example. A valve-operating mechanism for an internal combustion engine in which the intake and exhaust valves each have a dual structure is proposed in Japanese Patent Laid-Open Publication No. 7-77018, for example.
Japanese Patent Laid-Open Publication No. 2005-23831 describes a structure in which intake valves are disposed in the cylinder head, a recess is formed in a portion of the top of the piston so as to face one of the intake valves, and the space between the cylinder head and the top of the piston excluding the recess forms a squish area that has a large surface area. Such a large squish area facilitates the generation of squish and improves combustion.
The exhaust valve is not described, but the exhaust valve and intake valves ordinarily open in an overlapping state in the vicinity of top dead center in the exhaust stroke. In this overlapping state, fresh air that has been momentarily drawn in has a scavenging effect whereby residual gases are driven out, but adequate scavenging effect cannot be performed due to the inertia of the air intake and exhaust gas as the engine speed increases. The scavenging state is expressed as ratio of the mass of the fresh air remaining in the cylinder prior to combustion relative to the mass of the fresh air fed into the cylinder in a single cycle, i.e., is expressed as the trapping efficiency. Since the trapping efficiency directly affects engine performance, it is preferable to perform adequate scavenging while expanding the above-described squish area.
Japanese Patent Laid-Open Publication No. 2000-73803 describes a configuration in which the exhaust valve and intake valve are set in an overlapping state by closing the exhaust valve after air intake top dead center and opening the intake valve prior to air intake top dead center when the engine is under a high load; and the exhaust valve and intake valve are set in a non-overlapping state by closing the exhaust valve prior to air intake top dead center when the engine is under a low load.
Under a high engine load, the exhaust valve and intake valve are in an overlapping state, but since the amount of overlap is small, it is difficult to reliably carry out scavenging when the engine is operating at high speed.
In view of the above, it is possible to consider increasing the overlap amount and the valve lift distance. However, the intake and exhaust valves are liable to interfere with the top of the piston, and valve recesses in the top of the piston become necessary. Accordingly, the surface area of the squish area is reduced.
Japanese Patent Laid-Open Publication No. 7-77018 describes a configuration in which the intake valve is composed of an outer valve and an inner valve movably inserted inside the outer valve, a port is opened and closed by the outer valve, and a vent disposed in the outer valve is opened and closed by the inner valve. The outer valve is opened and closed by a rocker arm, and the inner valve opens when the interior of the combustion chamber has reached a prescribed negative pressure. The exhaust valve has the same structure as the intake valve.
The timing for opening and closing the inner and outer valves of the intake valve is not described, and the overlapping state of the intake and exhaust valves in the vicinity of top dead center of the exhaust stroke is not clear.
In view of the above, there is a need to expand the squish area as well as more reliably perform scavenging in the vicinity of the exhaust stroke in the high-speed region of an internal combustion engine.
A valve-operating mechanism for an internal combustion engine in which the intake and exhaust valves are radially disposed about a point on the center axis of the cylinder is proposed in Japanese Patent Laid-Open Publication No. 59-37213, for example.
Japanese Patent Laid-Open Publication No. 59-37213 describes a configuration in which the valve stems of two intake valves and two exhaust valves are radially disposed in the cylinder head about the center axis of the cylinder.
For example, the two intake valves are mounted in the cylinder head at a prescribed valve-included angle. Therefore, when the two intake valves open simultaneously, the outside diameter of the valve head and the lift distance are limited so that the intake valve does not interfere with the cylinder head, and it is difficult to increase the intake amount.
In view of this situation, there is a need to increase the amount of air intake in an internal combustion engine.
A valve-operating mechanism for an internal combustion engine that allows high speed rotation is proposed in Japanese Patent Laid-Open Publication No. 2000-297655.
The engine described in Japanese Patent Laid-Open Publication No. 2000-297655 has a crankshaft and camshaft rotatably mounted in the crankcase, a timing drive gear mounted in the crankcase, and a timing driven gear mounted on the camshaft. An idler gear meshes with the timing drive gear and timing driven gear. The cylinder head has intake valves and exhaust valves, valve springs that urge the intake and exhaust valves in the closing direction, rocker arms that press against the ends of the intake and exhaust valves to open and close the valves, and pushrods disposed between the rocker arms and the cams of the camshaft.
The idler gear is disposed on the cylinder head side of the crankshaft, and the camshaft can therefore be disposed on the cylinder head side. Therefore, the weight of the pushrod is reduced in an amount commensurate with the shortened pushrod, and the engine can be operated at higher speeds.
With the valve-operating mechanism for the engine described above, an increase in speed is restricted by resonance in the valve springs in the high-speed region or by other factors because the structure is one in which the intake and exhaust valves are closed by the valve spring.
In order to allow the camshaft to rotate, space for accommodating the idler gear must be provided inside the crankcase because of the added idler gear. This increases the size of the crankcase and consequently leads to an increase in the size of the engine. The complexity of the internal structure of the crankcase is increased and a structure for supporting the idler gear in the crankcase is required.
In view of the above, there is a need to increase the speed and reduce the size of an internal combustion engine, and make the structure more compact and simple.